Electronic Power Systems Tradeoffs Between Single Phase and Three-Phase Power

ELECTRONIC POWER SYSTEMS -
TRADEOFFS BETWEEN SINGLE-
PHASE AND THREE-PHASE POWER
MODULES
WHITE PAPER: TW0057Executive Summary
Modern Electronic Systems are quite often powered from a three-phasepower source. While utilizing power modules that operate directly fromthree-phase power might seem to provide optimal simplicity and flexibility,the added complexity required to realize three-phase power factorcorrected circuitry usually negates any potential savings. This papercompares single-phase and three-phase circuits from the component count,stress level and complexity standpoints. It demonstrates that when theseitems are taken into account, quite often the best choice is a combinationof single-phase modules, configured to balance individual power linephases.
2Introduction
In order to maximize power transfer while minimizing conductor volume, utility power is generallysupplied in a three-phase manner, either with or without a neutral connection (Figure 1). Note, when aNeutral is involved, it is generally connected to the installation's safety ground somewhere close to theinstallation's power entry point.
Figure 1 - Common Three Phase Power Sources
Common voltages for three phase systems in North America include 4-wire Delta Systems with a phase-to-phase voltage of 208VAC/60Hz, or 480VAC/60Hz. It is also common to see 480VAC/60Hz 5-wire WYEsystems with a phase-to-neutral voltage of 277VAC, or 208/60Hz 5-wire WYE systems with a phase-to-neutral voltage of 120VAC. In other parts of the world, typical power line voltages include 50Hz 5-wireWYE systems with a phase to phase voltage of 398VAC and a phase-to-neutral voltage of 230VAC, or aphase-to-phase voltage of 380V with a phase-to-neutral voltage of 220V.
Figure 2 presents the phase voltages for a 208VAC Delta System.
Figure 2 - Individual Phase Voltages versus Time for a 208VAC/60Hz System
Power conversion systems will typically be designed to draw power evenly from each phase so as not tooverload wiring, distribution transformers or circuit breakers. Managing the loading of the three phasescan be done by connecting apparatus separately to each phase in such a way that loading is balanced, or
3by using power converters that connect to all three phases simultaneously, automatically providing phasebalance. Figure 3 illustrates the two alternate methods.
Figure 3 - Power Conversion Converters with Single-Phase and Three-Phase Input
Single-Phase Input Power Modules
Single-phase modules provide the benefit of simpler and more efficient power conversion circuits, but inorder to fully balance line currents, power converters must be implemented in multiples of three. Findingthe right fit between individual power module size and overall system load can sometimes be challenging.If the modules are feeding a common load, then power-sharing circuitry must be employed betweenmodules so as to assure equal power drawn from each phase. Power sharing circuits may take the formof a "Droop" share circuit, where small, non-dissipative synthetic impedances are added in series witheach module's output. This will force modules to share current as if one unit's current is greater than theothers, its output voltage will drop until its load current matches other units in the system. Alternately,single wire, forced current sharing may be employed, where a separate wire is used to exchangeinformation between modules and the modules will reprogram their output voltage to enable currentsharing.
Three Phase Input Modules
Three-phase input modules provide the benefit of easing phase loading management, where the numberof supplies is not important. (If fault tolerant redundancy is required, then the minimum number ofmodules required is two.) However, three-phase modules come at the cost of added complexity andreduced efficiency and reliability at the power module level. If the modules feed a common load, power-sharing circuitry is still desired so as to assure balanced stresses on modules connected in parallel.
Comparison of Approaches
Let us examine some of the reasons that three phase modules are less efficient and reliable. Powerconversion efficiency in switching power supplies is a function of the number of switches required toimplement the converter, and the voltage and current... [download for more]